The present invention relates to a method according to the preamble of the appended claim 1.
In plants requiring high electric power, the employed power conductors are typically conductor rails made of copper or copper alloy. The conductor rails are long and typically include clamp joints created by means of fastening means, such as bolts. Usually the joints are overlapping joints, in which case through the pieces to be joined together, there is drilled at least one hole in the transversal direction. The holes are arranged to match, and therethrough there is inserted a fastening element, such as a bolt, which is tightened by means of a counterpart, such as a nut, so that the joint surfaces of the pieces to be joined are pressed together. While applying a bolt joint, the conduction of the electric current depends on the mechanical contact between the joined pieces. An effective and safe power conduction is secured only when the mechanical joint is in order. Clamp joints are typically oxidized in the course of time, in which case in the junctions there are created transfer resistances that consume an unnecessary high amount of electric power. Heavily power-consuming units for instance in the metallurgic industry are, among others, electrolytic plants and process-metallurgic electric furnaces. Owing to a weakening in the electroconductive capacity of bolt joints and to the creation of transfer resistances, remarkable economical losses are caused because of an increased energy consumption. Moreover, reparations of the joints, where even ten-year-old joints should be renewed, result in high work expenses.
The object of the invention is to realize a method and a joint whereby the drawbacks of the prior art can be avoided. Another object of the invention is to realize a method whereby it is possible, apart from making new joints, also repair old joints.
The invention is characterized in what is specified in the appended claims.
The arrangement according to the invention has several remarkable advantages. By applying the method according to the invention, there are achieved extremely durable and long-lasting joints with an electroconductive capacity that remains excellent. The method according to the invention is fairly easily applied in connection with different conductor rail arrangements. The joint is quickly made, and the method can be applied in various different installation conditions. By means of an embodiment of the method according to the invention, existing mechanical joints that are in poor condition can be easily repaired. By employing layer foil as the soldering/brazing agent, there is achieved an extremely advantageous soldering/brazing agent combination, which is easily applicable to use. When the method according to the invention is applied in repairing old joints, the old bolt joint can be left in place and bypassed by using a conductive element, which is for instance hard soldered on both sides of the old junction.
When the joining materials required for a diffusion joint are used in sufficiently thin and optimized layers, it is possible, even at low working temperatures, to obtain, by means of diffusion mechanisms, in the final joint structure such solid phases that have a melting temperature even hundreds of degrees higher than the melting temperature of the original joint material. Thus the joint is in a way self-correcting, because it endures higher temperatures than a pure joint material would endure. The heat required by the diffusion mechanisms between the joint material and the objects to be joined can be brought in the process by heating with a heating device, for example by a liquid gas burner. Other suitable heating arrangements, such as induction heating, can also be used for heating the junction area. On the surface of the soldering/brazing agent foil and/or at least in on of the surfaces to be joined, there is applied a layer of tin (Sn) prior to making the joint. The feeding of tin lowers the temperature required for making the joint. In addition, the oxidation of the surfaces to be joined is avoided, and protective gas arrangements are not needed in connection with the making of the joint. In order to start the phase transition reactions and to achieve an optimal structure in the joint seam, it suffices to have a tin layer of a few micrometers as the surface layer of the soldering/brazing agent in between the Ag+Cu center layer and the object to be joined. The technique according to the method is not critical for the Ag+Cu composition, in which case even essentially pure Ag foil can be used. The creation of the joint takes place rapidly as a result from the molten and solid material diffusion and the successive phase transition reactions, even at relatively low temperatures. By applying a preferred embodiment of the method of the invention, soldering/brazing agents resistant to very different oxidizing conditions have been created. With respect to corrosion taking place in the working conditions, the tin in the created mosaic gold is not harmful, because it is not sulfatized in the same fashion as zinc and copper. As for the silver that is dissolved in the phases of the junction seam, it helps to improve the corrosion resistance of the mosaic gold. By applying the method, there are obtained extremely good joints that maintain their electroconductive capacity and are well suited to the conductor rail arrangements in plants that use an extremely high electric power.